The Auroras

The “northern lights” (Aurora borealis) as well as the “southern lights” (Aurora australis) are upper-atmosphere phenomena of astronomical origin. The auroras center around the magnetic (not the geographical) poles of Earth, which explains why, in the Western Hemisphere, they have been seen as far to the south as New Orleans and Florida, while the equivalent latitude in the Eastern Hemisphere never sees an aurora. The northern magnetic pole happens to be in the Western Hemisphere.

The lower limit of an aurora is at about 50 mi (80 km). Upper limits have been estimated to be as high as 400 mi (640 km). Since about 1880, a connection between the auroras on Earth and sunspots has been suspected and has gradually come to be accepted. It was said that the sunspots probably eject “particles” (later the word electrons was substituted), which on striking Earth's atmosphere cause the auroras. But this explanation suffered from certain difficulties. Sometimes a very large sunspot group on the Sun, with individual spots bigger than Earth itself, would not cause an aurora. Moreover, even if a sunspot caused an aurora, the time that passed between the appearance of the one and the occurrence of the other was highly unpredictable.

This problem of the time lag is, in all probability, solved by the discovery of the Van Allen belt1, a double layer of charged subatomic particles around Earth. The inner layer, with its center some 1,500 mi (2,400 km) from the ground, reaches from about 40°N to about 40°S and does not touch the atmosphere. The outer layer, much larger and with its center several thousand miles from the ground, does touch the atmosphere in the vicinity of the magnetic poles.

It seems probable that the “leakage” of electrons from the outer Van Allen layer causes the auroras. A new burst of electrons from the Sun seems to be caught in the outer layer first. Under the assumption that all electrons are first caught in the outer layer, the time lag can be understood. There has to be an “overflow” from the outer layer to produce an aurora.

1. Named after the American physicist, James Alfred Van Allen (1914–2006), who discovered the broad bands of intense radiation surrounding Earth in 1958.